Process and apparatus for converting a solution or suspension into a dried particulate, granulate product

ABSTRACT

A mixer-granulator, preferably a spiral path granulator, is combined with a pneumatic conveyor dryer in a process for converting a solution or suspension into a dried particulate product. In a preferred embodiment, the particles formed in the mixer-granulator show a non-equilibrium moisture distribution with an interior part of lower moisture content and a surface part of higher moisture content.

BACKGROUND OF THE INVENTION

The present invention relates to a process and an apparatus forconverting a solution or suspension into a dried particulate, granulateproduct.

Pneumatic conveyor dryers (also called "flash dryers") are used forvarious industrial drying processes. Pneumatic conveyor dryers aredescribed, e.g., in Chemical Engineers' Handbook, fifth edition,McGraw-Hill Book Company, 20-55, and consist of a long tube or duct forcarrying a gas at high velocity, a fan or other means to propel the gas,a feeder for addition of the material to be dried and for dispersingsuch material in the gas stream, and a cyclone or other separation meansfor separating dried solid particles from the gas. In the usualembodiments of pneumatic conveyor dryers, either the feeder means or theinlet part of the tube or duct comprises means such as grinders,dispersion means, cage mills or rotating paddle means, which willdisintegrate particles or particle aggregates fed thereto so that theproduct being dried will usually have a smaller particle size than thefeed. Normally, particles dried in a pneumatic conveyor dryer have aparticle size of 50-300 μ.

It has now been found that a pneumatic conveyor dryer may advantageouslybe used for a drying process to obtain a dried particulate product ofgranular character in which, as a total result, the feed material is notdisintegrated to a smaller particle size, but rather "built up" andgranulated to obtain an end product consisting of discrete particles oflarger particle size than is normally produced in a pneumatic conveyordryer, the final particles consisting of a plurality of single fragmentsjoined together. One interesting embodiment of the process yields agranulate product consisting of particles of a very uniform particlesize and of a substantially spherical shape, the particles having a meanparticle size of 300-1000 μ, usually 500-900 μ. Such uniform granulateproduct of the particle size stated and of a free-flowing, non-dustingcharacter, attractive appearance and high bulk density, may be a mostuseful form of various commercial products, one interesting examplebeing the so-called "single cell protein" materials which may be used,e.g., in animal feed.

SUMMARY OF THE INVENTION

The process according to the invention for converting a solution orsuspension into a dried particulate product comprises mixing thesolution or suspension with recycled material, consisting of driedparticulate material of smaller particle size than the desired finalproduct, in a mixer-granulator to obtain particles of a higher moisturecontent than the desired final product, feeding the resulting particlesinto and conveying them through a pneumatic conveyor dryer without anysubstantial disintegration of the particles, classifying driedparticulate product discharged from the pneumatic conveyor dryer into afraction of desired particle size range, an undersize fraction, and, ifpresent, an oversize fraction, withdrawing particles of desired sizerange as final product, disintegrating oversize particles, if any, andrecycling the disintegration product together with undersize particlesinto the mixer-granulator.

DETAILED EXPLANATION OF THE INVENTION

The above-mentioned "mixer-granulator" is an apparatus which is capableof mixing a solution or suspension with a particulate product in such amanner that the resulting mixture is a particulate or "granular" productbuilt up from the solution or suspension and the particles. Variousmixer-granulators are known. For the purpose of the present invention,the mixer-granulator should preferably be one in which themixing-granulating process is completed within seconds or fractions ofseconds, and suitable mixer-granulators for use in the process of thepresent invention are high speed rotary mixer-granulators such as anupright spiral path granulator of the type discribed in the March/April1972 issue of "Phosphorus & Potassium", published by the British SulphurCorporation Ltd., Parnell House, 25 Wilton Road, London SW1V 1NH,England.

Such spiral path granulators are manufactured by Schuurmans & VanGinneken and are commercially available under the name "SCHUGI".

It is an essential feature of the present invention that the particlesformed in the mixer-granulator are fed to and conveyed through thepneumatic conveyor dryer without any substantial disintegration of theparticles, which is in contrast to the conventional pneumatic conveyordrying wherein disintegration normally occurs at one or more stations inthe particle flow. It is evident that a pneumatic conveyor dryersuitable for use in the process according to the present invention will,accordingly, not comprise disintegration means or other means which ineffect would result in a disintegration of the particles conveyedthrough the dryer. The feeding of the particles formed in themixer-granulator into the pneumatic conveyor dryer is advantageouslyperformed by allowing the particles to simply fall through a tube orduct into the drying tube or duct of the pneumatic conveyor dryeressentially by gravitational pull. As the particles leaving themixer-granulator have a relatively high moisture content and in oneembodiment of the invention, which will be explained further below, showa non-equilibrium moisture distribution with an interior of lessmoisture content and a surface part of higher moisture content, the tubeor duct through which the particles fall from the mixer-granulator intothe pneumatic conveyor dryer should be so designed that it presents nosubstantial obstruction to the free fall of the particles as, otherwise,the exteriorly moist particles would tend to stick to each other and tothe obstructing tube or duct parts. In practice, this means that thetube or duct, or at any rate the parts of the tube or duct which are sopositioned that they might obstruct the transportation of the particlesby gravitational pull, this being, at any vertical section of the tubeor duct, the lower part thereof, should be as close to vertical aspossible, taken along any generatrix. (The term "generatrix" is usedhere in a broad sense and comprises not only the proper generatrix of abody of revolution such as a tube, but also analogously covers a lineparallel to the longitudinal direction of e.g., a duct having arectangular cross-section. Moreover, it is to be understood that incases where e.g. a tube is curved, it will not be a proper body ofrevolution, but for the purpose of the present definition such curvedtube could be considered as consisting of several increments each beinga proper body of revolution, each small increment having its propergeneratrix). According to the present invention, the necessary freedomof obstruction to the gravitational pull transportation of the particlesfrom the mixer-granulator into the pneumatic conveyor dryer is obtainedwhen the lower generatrix of the tube or duct defines an angle withvertical of no more than 35°, more preferably less than 25°. The idealwould of course be a substantially vertical tube or duct from themixer-granulator to the pneumatic conveyor dryer, but as the drying tubeor duct of the pneumatic conveyor dryer is usually arranged in avertical or substantially vertical position, design limits regarding theheight of the installation will normally dictate an angle of the feedingtube or duct which is at least 10°, e.g. in the range of 10°-25°, and inpractical embodiments preferably 15°-20°. A relatively small angle, ofthe sizes mentioned above, between the drying tube or duct of thepneumatic conveyor dryer and the tube or duct through which theparticles pass from the mixer-granulator into the drying tube or duct isalso advantageous for the reason that the sudden shift in velocity anddirection encountered when the particles pass into the drying tube orduct will tend to break up any large lumps which might occasionally havebeen formed due to irregularities in the material flow.

The granulating effect achieved by the process according to the presentinvention is believed to depend at least partially upon a relativelyhigh degree of recycling. Normally, the relative amount of recycledmaterial to feed solution or suspension will be above 5:1, calculated ondry solids basis. Oversize particles discharged from the pneumaticconveyor dryer are disintegrated into smaller particles which arerecycled together with undersize particles, and depending upon theprocess parameters, it may be necessary or desirable to disintegrate andrecycle also part of the particles having the particle size range whichis specified for the desired end product. It will be within the realm ofthe skilled art worker to ascertain the degrees of recycling of thevarious fractions which will result in a stable process yielding aspecified desired product.

The suspension or solution fed to the mixer-granulator will have a totalsolids content in the range of 10-75% by weight, depending upon theidentity and character of the solid and the liquid. In most practicalapplications, the liquid will be water. If the solid is, e.g., a finelydivided mineral material such as clay, silicates, or the like betweenthe mixer-granulator (which builds up particles) the suspension may besufficiently dryable and suitable for the process of the presentinvention at high solids contents in the upper range of the intervalstated above, whereas with suspensions of various organic materials,solids contents in the lower range of the interval will often inherentlybe required. The total solids content of the recycled material willnormally be 80-100% by weight.

One important embodiment of the process of the invention, yielding theabove-mentioned substantially spherical particles, is characterized inthat the particle size distribution of the recycled material and therelative amounts of recycled material and solution or suspension are soselected that the particles formed in the mixer-granulator and passedinto the conveyor dryer show substantially a non-equilibrium moisturedistribution with an interior part of lower moisture content and asurface part of higher moisture content. In this embodiment of theinvention, a single small particle initially formed in themixer-granulator will be recycled through the system several times,e.g., about 20-25 times, before it is withdrawn as end product. Eachtime the particle emerges from the separation means of the pneumaticconveyor dryer, e.g. a cyclone, as an undersize particle, i.e. aparticle which will be classified in the undersize fraction by theclassifying means inserted in the circuit, e.g. a set of screens, itwill be passed into the mixer-granulator where a new layer of solutionor suspension is applied to the particle, whereafter the particle ispassed into the pneumatic conveyor dryer, and the newly applied moistlayer is dried. In this manner, this embodiment of the process of theinvention builds up the single particles of the granulated product byapplying a layer of moisture on the exterior of the particle and dryingthe layer in each cycle. In other words, in this embodiment of theinvention, the preliminary stages of the spherically shaped particlesare formed in the mixer-granulator, and it is believed that the shapingof the particles, resulting in the formation of spherical granules,continues through the entire pneumatic conveyor dryer, especiallycurvatures of the drying duct and the curved walls of the cyclone, theshapeability of the particles being probably dependent upon a relativelyhigh degree of plasticity of their moist exterior. The fact that theparticles show essentially a non-equilibrium moisture distribution withan interior part of lower moisture content and a surface part of highermoisture content is also believed to be the reason why it has at allbeen found possible to dry such unusually large size particles in apneumatic conveyor dryer, and furthermore, the non-equilibrium moisturedistribution with an exterior of relatively high moisture content alsopermits the use of drying air at a high temperature and hence theobtainment of good heat economy, without thermal deterioration of theproduct. The fact that the particles emerging from the mixer-granulatorin this embodiment show a non-equilibrium moisture distribution has, inconnection with the drying of a single cell protein suspension, beendemonstrated by withdrawing a sample of the particles produced in themixer-granulator. In the fresh sample, the particles are very plasticand sticky and may easily be compressed into a coherent mass, whereasafter standing for a few minutes, the particles have become non-stickyand appear dry. It will be understood that due to the relatively highmoisture and stickiness of the exterior of the particles in thisembodiment of the invention, the above-mentioned feature, that theparticles should fall from the mixer-granulator into the pneumaticconveyor dryer without any substantial obstruction, is highly preferred.

When the process of the invention is operated in accordance with theabove-mentioned embodiment, the total solids content of the feedsolution or suspension is suitably 15-35% by weight, while the totalsolids content of the recycled material is suitably 85-95% by weight.When the feed is a single cell protein suspension, the selected ratio ofthe relative amount of recycled material having a total solids contentin the stated range of 85-95% by weight to feed suspension having atotal solids content in the stated range of 15-35% by weight will be20-30:1 calculated on dry solids basis, and the selected particle sizedistribution of the recycled materials will be one in which at least 90%by weight of the recycled material has a mean particle size which is70-95% of the mean particle size of the final product, especially 80-90%of the mean particle size specified for the final product withdrawn.

In another embodiment of the process of the invention, the particle sizedistribution of the recycled material and the relative amounts ofrecycled material and solution or suspension are so selected that theparticles formed in the mixer-granulator show substantially anon-equilibrium moisture distribution with an interior part of highermoisture content powdered with fragments of lower moisture content. Alsohere, it is preferred that the total solids content of the feed solutionis 15-35% by weight, and that the total solids content of the recycledmaterial is 85-95% by weight. In this embodiment of the process of theinvention, the resulting granulate end product will show a lower bulkdensity than the substantially spherical product obtained in thefirst-mentioned embodiment and will consist of more porous andirregularly shaped particles of a size of 300-100 μ, typically 500-900μ. When, e.g., the product is a single cell protein for use in animalfeed, this configuration of the particles may be advantageous in that ityields, by admixture with other fodder components a stable product whichdoes not separate. In this embodiment, it is essential to recycle asufficient amount of sufficiently small particles to "powder"a liquiddroplet or a relatively wet aggregate of particles with small fragmentswhen building up the resulting particles to be conveyed through theconveyor dryer. When the feed is a single cell protein suspension, theselected ratio of the relative amount of recycled material having atotal solids content of 85-95% by weight to feed suspension having atotal solids content of 15-35% by weight is 10-20:1, calculated on drybasis, and the selected particle size distribution of the recycledmaterial is one in which at least 10% by weight, and especially 15-25%by weight, of the recycled material has a mean particle size which issmaller than 25% of the mean particle size of the final product.

In the process according to the present invention, the velocity of thedrying air in the pneumatic conveyor dryer is preferably high, andsuitable air velocities are 40-100 m/second measured at the throat orinlet section of the pneumatic conveyor dryer duct or tube and at theair inlet temperature, and, corresponding to this, 8-20 m/sec. in theconveyor dryer duct or tube proper. Especially preferred air velocitiesare 50-80 m/sec. in the throat section and, correspondingly, 10-16m/sec. in the duct.

The inlet temperature of the drying air is suitably 250-600° C, and fromheat economy considerations, it is preferred to keep a rather highdrying air inlet temperature such as 400-500° C.

In order to avoid an uneven velocity profile of the drying air and theconveyed particles in the pneumatic conveyor dryer duct, it is preferredto introduce the drying air into the throat section of the pneumaticconveyor dryer in a state of turbulent flow. Such turbulent flow of thedrying air introduced may be obtained by any suitable means, e.g. byobstructing the inlet air flow such as passing it into the throatsection of the pneumatic conveyor dryer through a right angle tubejoint.

British patent specification No. 1,119,078 discloses a process forgranulating a paste by spraying it onto fines recycled from a prioroperation, in which process the fines are evenly distributed over thecross sectional area of the upper end of an enclosure having asubstantially vertical axis of symmetry so that they fall by gravityalong paths which are substantially parallel to the axis of symmetry ofthe enclosure, and a gaseous flow stream is introduced into theenclosure in a direction substantially parallel to the path of thefines, and the paste to be granulated is projected onto the fines orcores during their fall by means of at least one spraying device whichis located on or in the vicinity of the axis of symmetry. Evidently,this process is very distinct from the process of the present inventionin essential respects and does not comprise the feature of generatingthe particles under vigorous mechanical influence, which is one of themain features of the present process and results in very stableparticles of high mechanical strength.

British patent specification No. 1,200,242 discloses a process for theagglomeration of a pulverulent material by introducing and dispersingthe pulverulent material into a turbulent stream of gas flowing along anarcuate path of relatively small cross-section such that the stream issubstantially linear, introducing an agglomerating fluid into the gasstream to form porous agglomerates and transferring the agglomeratesdirectly into a pneumatic conveyor dryer. A very essential differencebetween the process of British patent specification No. 1,200,242 andthe present process is that the feed material of the Britishspecification is the pulverulent material, in other words, the feed isintroduced in dry state and the liquid is used for agglomerating thefeed particles, whereas in the present process, the feed is derived fromthe fluid, that is, from the solution or suspension from which thegranulates are then generated. Also, the process of the British patentspecification uses a special mixing apparatus which is distinct from thespiral path granulator preferably used in the process of the presentinvention, and the British specification does not show or indicate thepreferred feature of essentially gravitational pull transportation ofthe particles from the mixer-granulator to the pneumatic conveyor dryer.The process of the British specification is especially designed forforming a readily soluble or dispersable agglomerates from powderedsolids which hydrate rapidly.

German Offenlegungsschrift No. 1,931,272 discloses a pneumatic conveyordryer having rotary paddles at its feed inlet for avoiding undesiredagglomeration of feed and/or caking at the interior dryer wall, and insome of the drawings, the feed tube or duct is so arranged thatessentially gravitational pull transportation of the feed into the dryeris possible, in other words, with a lower generatrix of the feed tube orduct defining an angle with vertical within about the same range as ispreferred according to the present invention. However, apart from thisisolated feature, the German Offenlegungsschrift does not contain anydisclosure resembling the essential features of the present invention.

The invention also relates to an apparatus for performing the processdescribed above, said apparatus comprising mixer-granulator means formixing a solution or suspension with a particulate material, a pneumaticconveyor dryer, means for feeding particles formed in the mixing meansinto the pnuematic conveyor dryer (said feeding means and said pneumaticconveyor dryer being free of particle disintegration means), means forclassifying particles discharged from the pneumatic conveyor dryer, andmeans for recycling particles from the classifying means into themixer-granulator. In preferred embodiments of the apparatus, themixer-granulator is a high speed rotary mixer-granulator, preferably aspiral path granulator, especially an upright spiral path granulator,and the tube or duct for feeding the particles from the mixer-granulatorinto the pneumatic conveyor dryer is preferably so designed that no partof its lower generatrix defines an angle with vertical of more than 35°.Preferably, the angle is less than 25°, especially 10°-25°, and mostpreferably 15°-20°. In a preferred embodiment, the apparatus comprisesmeans for obstructing the flow of the heating air upstream the throatsection of the pneumatic conveyor dryer so as to obtain turbulent flowof the heating air into said throat section, one practical embodiment ofsuch obstructing means being a right angle tube joint, preferably a Ttube section.

The process of the present invention has been performed in pilot scaleusing a single cell protein suspension ("single cell protein" heredesignates a material useful as protein source in human or animalnutrition and consisting of cells of yeast or mycelial fungi or ofbacteria) as feed suspension, and in the following, the invention willbe described in greater detail with reference to the treatment of asingle cell protein suspension, but it will be obvious to one skilled inthe art that the principle of the invention may also be applied to avariety of other suspensions or solutions which are to be converted intoa dried particulate granulate product.

EXPLANATION OF THE DRAWING

Reference is made to the drawing, which schematically represents anembodiment of an apparatus according to the invention.

A pneumatic conveyor dryer 1 comprises, as main components, a dryingtube or duct 16, a separating means 20 shown as a cyclone, fans 24 and52, and an air heating means 56. From a reservoir 2 for feed solution orsuspension, feed (exemplified as a single cell protein suspension) iswithdrawn and passed, by means of a pump 4, through a line 6 to amixer-granulator 8 shown as an upright high speed rotarymixer-granulator such as a spiral path granulator. In themixer-granulator, the single cell protein suspension is mixed with solidrecycled material, and the resulting moist particles fall through a tubeor duct 10 into a throat section 12 of the drying tube or duct 16 of thepneumatic conveyor dryer 1. Drying air is supplied by a fan 52 through aline 53 equipped with an adjustable damper 54 to a heating means 56(e.g., an oil or gas burner, an electrical heater or heat exchanger)from where it passes via a line 58 into an obstruction means orturbulency generator 14 shown as a T tube joint. The drying air passesinto the throat section of the conveyor dryer and conveys particles fromthe tube or duct 10 into and through the tube or duct 16. In theembodiment shown in the drawing, part of the tube or duct 16 is curvedas shown by the numeral 18. From the drying tube or duct, the mixture ofair and dried particles passes into the separating means 20 in which thedried particles are separated from the drying air. The separating means20 is shown as a cyclone, but also other separating means may be used,e.g., a bag filter. Through a line 22 equipped with an adjustable damper23, a fan 24 withdraws air from the cyclone, and through a line 26equipped with a damper 28, part of the air withdrawn by the fan 24 maybe passed into the mixer-granulator 8. From the separating means 20, thedried particulate product passes into a classifying means 30, e.g., avibrating screen set or, as shown, an inclined screen set comprisingscreens 32 and 34. The screen 32 retains oversize particles, but allowsundersize particles and particles having the particle size specified forthe desired end product to pass, whereas the screen 34 allows undersizeparticles to pass, but retains particles of the end product size.Undersize particles pass through a line 36 into a hopper 47. Oversizeparticles pass through a line 38 into a disintegrating means 40, shownas a pin mill, and the disintegrated product is passed into the hopper47. Particles of the desired particle size range pass through a line 42and are withdrawn as desired end product, as indicated by the arrow.However, by means of a product splitter 44, shown as an adjustabledamper, part of the particles of the size range specified for thedesired end product may be passed into the disintegrator 40 via a line46. From the hopper 47, the material to be recycled passes to a dosingmeans 48, e.g., a band conveyor or (as shown) a screw conveyor, fromwhich it is fed through a line 50 into the mixer-granulator 8. It is tobe understood that the process and the apparatus of the invention arenot to be limited to the flow sheet and apparatus schematically shown inthe drawing.

In the drawing, the lower part of a conical section 11 of the tube 10constitutes the part of the tube which defines the largest angle withvertical. The inclination of the lower generatrix (the said lowergeneratrix being shown by a line G in the drawing) constitutes thecritical parameter with respect to obstruction of the material flow fromthe mixer-granulator to the throat section of the pneumatic conveyordryer. In the drawing, the said lower generatrix is shown as having aninclination of 35° from vertical, but as stated above, it is preferredthat the said angle is smaller.

In preferred commercial scale apparatus of the invention, the dryingtube or duct of the pneumatic conveyor dryer will have a length of about7-20 meter, including the curved part thereof, if any. If a T tube jointis used as turbulency generator, such as shown in the drawing, its lowerpart may suitably be closed with a spring-loaded drop which may beadjusted to automatically drop any large lumps collecting in the joint,or to allow manual opening for withdrawal of any lumps and/orinspection. In the operation of the apparatus shown in FIG. 1, thedampers 23, 28, and 54 are preferably adjusted in such a manner that theabsolute pressure in such parts of the apparatus interior as thescreens, the hopper, the dosing means, and the mixer-granulator isbetween neutral and slightly reduced (e.g. between -20 and -30 mm H₂ O)in relation to the ambient pressure. In this connection, it may besuitable to recycle, through line 26, 0-15% of the total amount ofdrying air supplied.

In the operation of the apparatus shown in the drawing, the processconditions are adjusted to obtain a stable process with the desiredparameters, e.g. the parameters of the above-mentioned preferredembodiments, by adjusting suitable variables such as the rate of feedingfrom the reservoir 2, the rate of recycling from the dosing means 48,the proportion of particles of the size range specified for the desiredend product recycled through the line 46, and the mesh size of thescreens 32 and 34. The adjustment of such variables may be performed inknown manners, either manually, or automatically.

EXAMPLES

The following examples illustrate the invention:

EXAMPLE 1

A biomass consisting of a single cell protein slurry with 20% solidscontent was dehydrated and granulated in a plant as illustrated in thedrawing.

The mixer-granulator used was a Schugi continuous mixer manufactured bySchuurmans & Van Ginneken, Netherlands.

50 kg/h slurry were introduced into the mixer together with 270 kg/hrecycled material consisting of

a. 267.1 kg/h undersize with the following properties:

    ______________________________________                                        Moisture content:  10%,                                                       bulk density:      0.65 g/cm.sup.3,                                           mean particle size:                                                                              750 μ,                                                  fraction below 250 μ:                                                                         3%                                                         ______________________________________                                    

and

b. 0.4 kg/h oversize (above 2 mm) and 2.5 kg/h product having a particlesize in the range specified for the desired end product, both ground toa fraction with the following properties:

    ______________________________________                                        Moisture content:  15%,                                                       bulk density:      0.64 g/cm.sup.3 ,                                          mean particle size:                                                                              220 μ,                                                  fraction below 100 μ:                                                                         18%.                                                       ______________________________________                                    

The mixer-granulator-shaft was rotating at a velocity of 2800 rpm.

The velocity of the drying gas in the throat of the pneumatic conveyordryer was about 70 m/sec, at an inlet temperature of 500° C.

The final product withdrawn from the plant consisted of compact,spherical and smooth particles. The product was free-flowing andnon-dusting and had the following properties:

    ______________________________________                                        Moisture content:  12.5%, -bulk density: 0.65 g/cm.sup.3,                     mean particle size:                                                                              850 μ,                                                  fraction below 750 μ:                                                                         1%.                                                        ______________________________________                                    

The product was afterdried to a residual moisture content of 8% in aconventional vibrated fluidized bed using drying air at 200° C. Apartfrom the reduced moisture content, the product retained itsabove-mentioned properties.

EXAMPLE 2

The same plant, the same single cell protein slurry and the sameoperating conditions as described in Example 1 were used.

50 kg/h slurry were introduced into the mixer together with 142.5 kg/hrecycled material consisting of a. 115.9 kg/h undersize with thefollowing properties:

    ______________________________________                                        Moisture content:  11.5%, -bulk density: 0.53 g/cm.sup.3,                     mean particle size:                                                                              480 μ,                                                  fraction below 250 μ:                                                                         15%                                                        ______________________________________                                    

and

b. 3.6 kg/h oversize (above 2 mm) and 23 kg/h product having a particlesize in the range prescribed for the desired final product, both groundto a powder with the following properties:

    ______________________________________                                        Moisture content:  22%,                                                       bulk density:      0.52 g/cm.sup.3,                                           mean particle size:                                                                              200 μ,                                                  fraction below 100 μ:                                                                         20%                                                        ______________________________________                                    

The product withdrawn as final product consisted of porous, irregularlyshaped particles. It was rather free-flowing and non-dusting and showedthe following properties:

    ______________________________________                                        Moisture content:  21%,                                                       bulk density:      0.44 g/cm.sup.3,                                           mean particle size:                                                                              780 μ,                                                  fraction below 750 μ:                                                                         47%,                                                       fraction below 500 μ:                                                                         2%.                                                        ______________________________________                                    

The product was afterdried to a residual moisture content of 8% in aconventional vibrated fluidized ben using drying air at 200° C. Apartfrom the reduced moisture content, the product retained itsabove-mentioned properties.

Among the products prepared by the process of the invention, theproducts prepared by the embodiment exemplified in Example 1, i.e. agranulate product consisting of particles of a very uniform particlesize and of a substantially spherical shape, show unique properties andare believed to be novel.

These novel granulate products constitute an aspect of the presentinvention and may be defined as granulate products comprising granulesor nodules of dried single cell protein, said granules or nodules havinga mean particle size of 300-1000 μ, and a specific gravity of 1.4-1.8g/cm³, the bulk density of the granulate product being 0.5-0.7 g/cm³.

Among these novel granulate products, preferred products are products inwhich the granules or nodules are of a substantially spherical shape,and the mean particle size is 500-900 μ. These novel products accordingto the invention are remarkable in that they show surprisingly highmechanical strength combined with a very pronounced non-dustingcharacter. These properties are highly desirable in single cell proteinproducts, as the non-dusting character and the exceptional hardness andmechanical strength minimizes the risk that the product should becomedisintegrated by handling, which is especially valuable in connectionwith a single cell protein product, because single cell protein productsare inherently mucous membrane-irritating, for which reason single cellprotein product dust may be dangerous or at least objectionable topersons involved in handling the product and also may render the singlecell protein product objectionable to the animals which are to consumethe product.

In such granulate products according to the invention, the proportion ofparticles having a size of less than 300 μ may be kept very low and isusually less than 0.1% by weight of the product. With respect to theparticle size distribution within the particle size ranges of 300-1000μ, or, in a preferred product, 500-900 μ, this may be controlled byproper selection of the mesh sizes of the screens used in theclassification of the product, and in some cases, it may be desirable toprepare a product of a very uniform particle size, whereas for otherpurposes, a product with a somewhat less uniform particle size, withinthe ranges stated, may be preferred because of its higher bulk density.

The high mechanical strength of the product according to the inventionmanifests itself in various manners. As indicated above, onemanifestation of the mechanical strength is the resistance todisintegration by handling, and another manifestation is a highresistance to compression. Furthermore, when poured into water, theproduct prepared according to Example 1 will not become dissolved orsuspended, but will collect at the bottom and will retain itssubstantially spherical character even for prolonged periods such as oneweek. A certain degree of swelling will occur, and is estimated to be bya factor of about 1.2. The fact that the product is non-dusting, showshigh mechanical strength and retains its character when immersed inwater is believed to be very advantageous when the product is to be usedas constituent of fodder compositions, especially for ruminants.

We claim:
 1. A process for converting a solution or suspension includingrecycled material into a dried particulate product in a material cyclingsystem including a pneumatic conveyor dryer and a mixer-granulator, saidprocess comprising:a. mixing in said mixer-granulator, the solution ofsuspension with recycled material consisting of dried particulatematerial of smaller particle size than the desired final product, toobtain particles of a higher moisture content than that of the desiredfinal product, b. feeding the resulting particles into and conveyingthem through said pneumatic conveyor dryer without any substantialdisintegration of said particles, c. classifying dried particulateproduct discharged from the pneumatic conveyor dryer into a fraction ofdesired particle size range, an undersize fraction, and, if present, anoversize fraction, d. withdrawing particles of desired size range asfinal product, e. disintegrating oversize particles, if any, and f.recycling the disintegration product together with undersize particlesinto said mixer-granulator.
 2. A process according to claim 1, whereinpart of the fraction of desired particle size range is disintegrated andrecycled to the mixer-granulator together with undersize particles anddisintegrated oversize particles, if any.
 3. A process according toclaim 1, wherein a high speed rotary mixer-granulator is used.
 4. Aprocess according to claim 3, wherein the high speed rotarymixer-granulator used is an upright spiral path granulator.
 5. A processaccording to claim 1, wherein the particles discharged from themixer-granulator are passed from the mixer-granulator through a tube orduct to the pneumatic conveyor dryer essentially by gravitational pull.6. A process according to claim 5, wherein any part of the lowergeneratrix of the tube or duct defines an angle with vertical of no morethan 35°.
 7. A process according to claim 5, wherein the angle is lessthan 25°.
 8. A process according to claim 7, wherein the angle is10°-25°.
 9. A process according to claim 8, wherein the angle is15°-20°.
 10. A process according to claim 1, wherein the relative amountof recycled material to feed solution or suspension is above 5:1,calculated on dry solids basis.
 11. A process according to claim 10,wherein the total solids of the feed solution or suspension is 10-75% byweight, and the total solids content of the recycled material is 80-100%by weight.
 12. A process according to claim 1, wherein the particle sizedistribution of the recycled material and the relative amounts ofrecycled material and solution or suspension are so selected that theparticles formed in the mixer-generator and passed into the conveyordrier show substantially a non-equilibrium moisture distribution with aninterior part of lower moisture content and a surface part of highermoisture content.
 13. A process according to claim 12, wherein theparticles discharged from the mixer-granulator are passed from themixer-granulator through a tube or duct to the pneumatic conveyor dryeressentially by gravitational pull.
 14. A process according to claim 13,wherein any part of the lower generatrix of the tube or duct defines anangle with vertical of no more than 35°.
 15. A process according toclaim 12, wherein the total solids content of the feed solution orsuspension is 15-35% by weight, and the total solids content of therecycled material is 85-95% by weight.
 16. A process according to claim15, wherein the feed is a single cell protein suspension, and the ratioof the relative amount of recycled material to feed suspension is20-30:1, calculated on dry solids basis.
 17. A process according toclaim 16, wherein at least 90% by weight of the recycled material has amean particle size which is 70-95% of the mean particle size of thefinal product.
 18. A process according to claim 17, wherein at least 90%by weight of the recycled material has a mean particle size which is80-90% of the mean particle size of the final product.
 19. A processaccording to claim 1, wherein the particle size distribution of therecycled material and the relative amounts of recycled material andsolution or suspension are so selected that the particles formed in themixer-granulator show substantially a non-equilibrium moisturedistribution with an interior part of higher moisture content powderedwith fragments of lower moisture content.
 20. A process according toclaim 19, wherein the total solids content of the feed solution orsuspension is 15-35% by weight, and the total solids content of therecycled material is 85-95% by weight.
 21. A process according to claim20, wherein the feed is a single cell protein suspension, and the ratioof the relative amount of recycled material to feed suspension is10-20:1, calculated on dry solids basis.
 22. A process according toclaim 21, wherein at least 10% by weight of the recycled material has amean particle size which is smaller than 25% of the mean particle sizeof the final product.
 23. A process according to claim 22, wherein15-25% by weight of the recycled material has a mean particle size whichis smaller than 25% of the mean particle size of the final product. 24.A process according to claim 1, wherein the velocity of the drying airin the throat section of the pneumatic conveyor dryer is 40-100 metersper second, and the velocity of the drying air in the pneumatic conveyordryer duct is 8-20 meters/second.
 25. A process according to claim 24,wherein the velocity of the drying air in the throat section of thepneumatic conveyor dryer is 50-80 meters/second, and the velocity of thedrying air in the pneumatic conveyor dryer duct is 10-16 meters/second.26. A process according to claim 24, wherein the inlet temperature ofthe drying air is 250°-600° C.
 27. A process according to claim 26,wherein the inlet temperature of the drying air is 400-500° C.
 28. Aprocess according to claim 1, wherein the drying air is introduced intothe throat section of the pneumatic conveyor dryer in a state ofturbulent flow.
 29. The product produced by the process of claim
 1. 30.An apparatus for converting a solution or suspension into a driedparticulate product comprising a mixer-granulator means for mixing asolution or suspension with a particulate recycle material, anon-disintegrator pneumatic conveyor dryer, means for feeding particlesformed in the mixer-granulator means into the pneumatic conveyor drier,means for classifying particles discharged from the pneumatic conveyordryer for recovering a dried particulate product and particulate recyclematerial, and means for recycling particles from the classifying meansinto the mixer-granulator.
 31. An apparatus according to claim 30,wherein the mixer-granulator is a high speed rotary mixer-granulator.32. An apparatus according to claim 31, wherein the high speed rotarymixer-granulator is an upright spiral path granulator.
 33. An apparatusaccording to claim 30, wherein the means for feeding the particles fromthe mixer-granulator into the pneumatic conveyor dryer is a tube orduct.
 34. An apparatus according to claim 33, wherein any part of thelower generatrix of the tube or duct defines an angle with vertical ofno more than 35°.
 35. An apparatus according to claim 33, wherein theangle is less than 25°.
 36. An apparatus according to claim 35, whereinthe angle is 10°-25°.
 37. An apparatus according to claim 36, whereinthe angle is 15°-20°.
 38. An apparatus according to claim 30, comprisingmeans for obstructing the flow of the drying air upstream of the throatsection of the pneumatic conveyor dryer for providing turbulent flow ofthe drying air into said throat section.
 39. An apparatus according toclaim 38, wherein said obstructing means is a right angle tube joint.